Protein phosphorylation is one of the most fundamental signaling processes in eukaryotic cells and in human disease. Since phosphorylation of any site on a given protein can change the function or localization of that protein, understanding the "state" of a cell requires knowing the phosphorylation state of its proteins. Identification of key phosphorylation sites and generation of phospho-specific antibodies have proved to be critical for basic research and for clinical diagnosis, especially relating to cancer biology. It is essential to have an expert-curated web-accessible unified source of information on such sites and phospho-specific antibodies, in order to facilitate use of such antibodies in basic and pre-clinical research. The MPR (Mammalian Phosphorylation Resource) is a unique resource for this information and for related information on phosphorylation biology.